The luminescent bodies of organic electroluminescent elements (hereinafter may be referred to as “organic EL elements”) can be formed in a planar shape, and the color of light emitted therefrom can be white or a color close to white. Therefore, it is contemplated that such organic EL elements are used as light sources of lighting devices for illuminating areas in living environments etc. or in applications for backlights of display devices.
As an example of the organic EL elements used in lighting applications, white organic EL elements are being manufactured. Many of such white elements are of the stacked or tandem type in which luminescent layers that emit light with complimentary colors are stacked. The stacks of such luminescent layers are mainly stacks of yellow/blue luminescent layers or green/blue/red luminescent layers.
However, currently known organic EL elements are unsatisfactory in terms of efficacy for the aforementioned lighting applications. Therefore, when an organic EL element is used as a surface light source, it is required to improve its light extraction efficiency. For example, although the luminescent efficiency of the luminescent layer itself of an organic EL element is high, the amount of light is reduced due to, for example, interference in the layers in the stacked structure constituting the element when the light passes through the stacked structure and is emitted to the outside. Therefore, it is required to reduce such a loss of light as much as possible.
One known method for improving the light extraction efficiency of organic EL elements is to provide various structures on their light-extraction substrates. For example, it is proposed to provide prisms containing a fluorescent compound on the light-emitting surface of a light source device (Patent Document 1). It is also proposed to provide a micro-lens array on the light-emitting surface of a light source device (Patent Document 2). With these structures, light can be gathered in a favorable manner, and the efficiency is thereby improved. As another example of means for improving the light extraction efficiency of an organic EL element, Patent Document 3 discloses that a light diffusing medium is provided on the light-emitting side of the organic EL element to improve the overall brightness.